1. Field of the Invention
The present invention concerns a multilayer cellular mobile radio network. A network of this kind comprises first base stations defining respective macrocellular coverage areas (macrocells) and second base stations defining respective microcellular coverage areas (microcells). The term "microcell" covers both the picocell and the microcell concept. Microcells have a small diameter and are used to meet local demand, for example to provide service in geographical areas with a relatively high concentration of mobiles, such as road junctions, shopping streets, airports and shopping centers in the case of microcells and floors of a building in the case of picocells, or to provide service in boxed in areas where radio access is difficult. Macrocells have a substantially greater diameter and provide coverage of areas with a lower density of mobiles and more extensive radio access. A microcell can be "internal", in which case it is part of a macrocell which covers it, the term "macrocell" then being used to denote the outermost layer of a cellular structure, or "external", in which case it is not covered by any macrocell.
The cellular architecture of a mobile radio network gives rise to the problem of frequency re-use, that can be formulated generically as follows: a frequency used in a given cell typically cannot be re-used in a cell immediately adjacent said given cell. If macrocells and microcells co-exist, frequency planning usually proceeds in two stages, respectively relating to the macrocells and to the microcells. Planning problems arise in particular if new microcells are added to meet local demand.
2. Description of the Prior Art
The first planning stage is concerned only with the macrocells. These share a given frequency spectrum and a geometrical pattern of frequency re-use is defined and this defines a minimal distance between two macrocells that can use the same frequency. In a combined time-division multiple access (TDMA) and frequency-division multiple access (FDMA) radio system, of the GSM or DCS-1800 type, the frequency hopping technique offers a statistically significant improvement in traffic link quality by enabling the use of a limited number of identical frequencies in neighboring cells, as described in "The GSM System for Mobile Communications" written and published by M. B. Pautet and M. Mouly, 1992 edition, pages 218-223.
The second planning stage concerns the microcells. Two techniques are used.
The first technique dedicates a first portion of the total spectrum available for use by microcells only. In this case, this first portion is strictly reserved for microcells and cannot be used by macrocells. A second portion, complementary to this first portion throughout the available spectrum, is dedicated to macrocells and cannot be used by microcells. Using this technique, in an area covered by a macrocell containing only a few microcells, spectrum sharing can seem unfavorable for said macrocell. The microcells, used for a small geographical area, are called upon to support calls less often than the macrocell and have a large portion of the total available spectrum for this small area, while the macrocell alone is used for a large geographical area, and for this has only a relatively small portion of the total spectrum. On the other hand, in an area covered by a macrocell containing many microcells, the sharing of the spectrum is unfavorable to the microcells. The microcells are used for a large geographical area and have a relatively small portion of the total spectrum available for this large area, while the macrocell is used for a small geographical area and has a relatively large portion of the total spectrum for this.
The second technique uses in a given microcell a portion of the spectrum corresponding to a portion of the spectrum used by macrocells very far away. In this case, the total capacity of the macrocells, in terms of the spectrum, is not affected by spectrum sharing. This second technique, as briefly described in European patent application EP-A-2 697 390 (ERICSSON), is nevertheless subject to the limitation that it can be used only if a geometrical pattern of frequency re-use is defined that limits the available spectrum band for each macrocell.
This is explained with reference to FIG. 1, which shows a pattern of frequency-reuse with seven cells. Using this pattern, the total spectrum available is divided into seven different frequency sub-bands A, B, C, D, E, F and G. Each macrocell, represented as a hexagon, is contiguous with and surrounded by six macrocells using six respective frequency bands each separate from its frequency band. For a given microcell belonging to one of these macrocells, a geographical pattern of frequency re-use of this kind rules out the use by this microcell of any of the frequencies of the total spectrum available, since this frequency is used by a macrocell the coverage area of which is at a distance less than the diameter of a macrocell from said microcell, and would therefore cause interference. To solve this problem it would be necessary to extend the pattern to a very large number of cells so that each macrocell is not contiguous with and surrounded by macrocells that together define the totality of the available spectrum. This would go against optimizing spectrum use by the macrocell, since the portion of the spectrum assigned to each macrocell is small.
Patent application GB-A-2 260 879 describes a method of assigning a control channel to a microcell to be installed. In this method, the microcell to be installed measures the power level of the traffic channels of surrounding macrocells and selects the traffic channel on which the power level is the lowest, and this channel is used as a control frequency in the microcell to be installed. This method has the major drawback of using as the control channel in the microcell to be installed a frequency identical to a frequency of a traffic channel in a macrocell that is by definition far from the micrccell to be installed. This rules out use of this same frequency as a traffic channel in said microcell to be installed and therefore places a limitation on the capacity of the network.